1. Field of the Invention
The present invention relates to a method of equalizing resistances of heating elements of a thermal head of a thermal printer. The present invention also relates to a device for a thermal printer which equalizes resistances of heating elements of a thermal head. The present invention relates more particularly to a device and a method for equalizing resistances of the heating elements by using a resistance trimming process wherein the heating elements are preliminary heated by trimming energy.
2. Related Art
A thermosensitive color recording material has been suggested, for example, in Japanese Laid-open Patent Application 61-213169, which has those thermosensitive coloring layers for yellow, magenta and cyan which are laminated or formed on a supporting material in this order from the outside. In this type of recording material, the heat sensitivities of the thermosensitive coloring layers (hereinafter referred to as coloring layers) become lower with the distance from the outside surface. Furthermore, the coloring layers have properties that each coloring layer is optically fixed by electromagnetic rays of a respective specific wave length range. Therefore, recording of a full-color image on the above-described thermosensitive color recording material is performed in the order from the top or outermost coloring layer to the inner coloring layer, while optically fixing the just recorded coloring layer prior to recording the next coloring layer, so as to avoid undesirable double recording.
The thermal printer includes a thermal head having a plurality of heating elements which are connected in parallel to one another and arranged in an array. The thermal head gives a variable amount of heat energy to the color thermosensitive recording layer depending on the sensitivity of the color recording layer to be color developed. Specifically, a bias heat energy is first applied for heating the thermosensitive color recording material up to such a temperature above which a predetermined color begins to be developed in the corresponding color recording layers, the amount of bias heat energy is constant and determined according to the sensitivity of each color recording layer. Next, a variable amount of gradation heat energy necessary for developing the color at a desirable density is applied.
To reproduce a fine gradation, it is necessary to accurately control the amount of gradation heat energy. In general, the heating elements are activated or power is conducted for about several milliseconds or several tens of milliseconds for the bias heating. On the other hand, the conduction time of the heating elements is controlled at an accuracy of several micro seconds or several tens of micro seconds.
In spite of such a fine control of heating or conduction time of the heating elements, the consequent image cannot exactly reproduce the desired fine gradation unless all the heating elements of the same thermal head have a completely uniform resistance value. However, it is generally assumed that the heating elements have a variation of about 5% in resistance. For this reason, the printed images tend to have troubles, such as chromatic unevenness, due to the unevenness of the thermal elements.
To avoid such troubles, a thermal printer has been known, for example, from Japanese Laid-open Patent Application No. 2-248262, wherein resistance values of all the hundreds of heating elements of the thermal head are measured, and correction data is calculated based on the results of measurement, so as to correct image data by the correction data. Another thermal printer as disclosed in Japanese Laid-open Patent Application No. 2-292060 interpolates density correction pulses between gradation pulses so as to compensate for the chromatic unevenness caused by the unevenness in resistances of the heating elements.
However, in order to interpolate the density correction pulses, an additional pulse generation circuit for generating the correction pulses is necessary, which increases the cost of the thermal printer. Moreover, interpolation of the density correction pulses increases the printing time a, compared with the case where no correction pulse is interpolated.
Because an enormous operation is necessary for directly correcting the image data by the correction data, the former method needs a high speed calculating circuit so that the cost of the thermal printer also increases. Besides that, because the operation of the image data amplifies quantizing distortion, printed images contain pseudo outlines thereby lowering the quality of printed image.